This invention relates to a radiation shielding device for nuclear facilities, and more particularly to a radiation shielding device mounted at an opening formed in a radiation shield for a pressure vessel of a nuclear reactor to permit a nozzle and a pipe joined by welding to the pressure nozzle to extend therethrough which device can be freely brought to open and closed positions to prevent the whole-body of an inspector inspecting the welds from exposure to radiation.
With the progress of nuclear fission within a pressure vessel of a nuclear reactor, various kinds of radiation, such as alpha rays, beta rays, gamma rays and neutrons, are emitted by fission products. It is a well known fact that exposure to radiation can have toxic effects on human beings and can cause leukemia, tumors, other forms of cancer, cataract, sterility and the birth of freaks.
In order to preclude human beings from radiation exposure, the outer surface of the pressure vessel of the nuclear reactor is shielded by structures made of concrete, iron and lead. The pressure vessel has a nozzle and a pipe connected to the nozzle for introducing cooling water into and releasing steam from the pressure vessel. The nozzle is joined by welding to the pressure vessel, and the pipe is joined by welding to the nozzle. Inspection of the welds is carried out at regular intervals to avoid leakage of steam or gas therethrough.
The International Commission on Radiological Protection (I.C.R.P. for short) has recommended that the following guideline radiation figures be adopted for occupational exposures in order to prevent acute effectiveness of exposure to radiation and to set a tolerable level for late effected risk for workers engaged in the nuclear industrial shop:
1. An integrated dose: 5 (N-18) rem should be set as a limited amount of radiation to be accumulated in one year for persons of an age N; PA1 2. a maximum permissible dose (MPD for short) for various parts of the human body: a strict standard should be set for exposure of the whole human body to radiation (a limit of 3 rem per one quarter year or 5 rem per one year) as compared with exposure of a part (the hand, foot, head, etc.) of the human body in such a ratio that the permissible dose for the former is one fifth to one ten-oddth the dose for the latter; and PA1 3. A dose limit of planned special exposure to which workers in a nuclear-industrial shop may be subjected: a figure twice the figure set as MPD is permitted, but this figure should not be exceeded by the integrated dose of paragraph (1).
The inspectors who carry out the inspection of the welds at regular intervals of time should be protected from radiation damage such that the figure for permitted radiation which they receive should be below the aforementioned levels. The manner in which the regular inspection of the welds has hitherto been carried out will be described along with the construction of a radiation shielding device of the prior art used for preventing leakage of radiation.
There is provided an outer layer for shielding radiation which encloses the pressure vessel of the nuclear reactor in uniformly spaced relationship with the outer surface of the pressure vessel. Interposed between the outer layer and the pressure vessel is an inner layer for insulating heat which encloses the entire outer surface of the pressure vessel in spaced relationship.
A nozzle projecting outwardly from the pressure vessel and a pipe connected to the nozzle extend through openings formed in the inner layer and the outer layer.
A removable heat insulator consisting of a plurality of pieces to facilitate removal is detachably attached to an edge of the inner layer which defines the opening formed therein, so that a weld formed between the pressure vessel and the nozzle can be inspected readily.
To enable the removable heat insulator to be detached piece by piece from its position and conveyed to outside when inspection of the weld is performed, the material of the outer layer is removed at a portion thereof where the opening is formed, and a shielding frame is secured to walls of the opening, to provide a manhole of a size large enough to enclose the nozzle and the pipe and to permit the operator to enter therein to convey the heat insulator therethrough or to carry out weld inspection. A plurality of shielding blocks made of a radiation shielding material are removably attached to the shielding frame in such a manner that the shielding blocks close from outside an open space between the shielding frame and the pipe.
Ultrasonic inspection techniques are usually employed in inspecting the welds formed between the pressure vessel and the nozzle and between the nozzle and the pipe. Ultrasonic flaw detection is carried out while the inspector performing the inspection keeps a probe in direct contact with the welds to be tested for producing ultrasonic waves from the probe.
Generally the nozzle projects horizontally from the pressure vessel, so that it is necessary for the operator to move in the manhole defined by the shielding frame disposed horizontally while manually keeping the probe in contact with the welds to be tested.
To this end, the shielding blocks are first opened and then the heat insulator is removed piece by piece from its position by being conveyed through an open space between the pipe and the shielding frame.
After the welds have been exposed in this way, the inspector has access through the manhole to the welds and manually moves the probe along the welds so as to detect flaws, if any.
Of all the kinds of radiation released from the core of the reactor, gamma rays are predominantly large in amount as compared with alpha rays, beta rays and neutrons when regular inspection is carried out by shutting down the reactor. Gamma rays, which are a sort of electromagnetic waves, have the characteristic of decaying in inverse proportion to the square of the distance from the source of radiation. Assuming that the dose of radiation detected in the regular inspection position when the reactor is shut down is 4 roentgens per hour, this will be 4 rem per hour because one rem or gamma rays is the same as one roentgen of them. If the aforementioned recommendation made by the I.C.R.P. is observed strictly, an inspector would be able to carry out an inspection operation only for 45 minutes per one quarter year or 1 hour and 15 minutes per one year as long as exposure of the whole-human body is concerned.
Thus it becomes necessary to rely on human wave tactics in performing regular inspection of the welds. The prior art method of inspection has had the disadvantages of increased personnel expenses and other expenses, enhanced trouble caused by the need to provide protection from radiation damage, and some times a delay in carrying out the required inspection.